GUI bugfix (histogram) + touchup

- Fix the Histogram background and grid lines
- Add 1 pixel of empty space between in the Histogram panel
- Add a light grey color border for Gtk::DrawingArea oject, for those
using the render_frame method
- In the Toolbox, make the nested MyExpander and Gtk::Frame lighter, and
adjust the Gtk::DrawingArea background color accordingly
- All added class to the MyCurve has been removed, so that the
Gtk::Drawing object's style is used
- The Retinex Settings' expander is now level 2 for GUI consistency

rtgui/retinex.cc

@@ -328,6 +328,7 @@ Retinex::Retinex () : FoldableToolPanel(this, "retinex", M("TP_RETINEX_LABEL"),
     settingsVBox->pack_start (*medianmap);
     medianmap->show ();
     expsettings->add(*settingsVBox);
+    expsettings->setLevel(2);
 
     neutrHBox = Gtk::manage (new Gtk::HBox ());
     neutrHBox->set_border_width (2);